Renin is a proteolytic enzyme synthesized and stored principally in a specific part of the kidney called the juxtaglomerular apparatus. Any of three different physiologic circumstances may cause the release of renin into the circulation: (a) a decrease in the blood pressure entering or within the kidney itself; (b) a decrease in the blood volume in the body; or (c) a fall in the concentration of sodium in the distal tubules of the kidney.
When renin is released into the blood from the kidney, the renin-angiotensin system is activated, leading to vasoconstriction and conservation of sodium, both of which result in increased blood pressure. Renin acts on a circulating protein, angiotensinogen, to cleave out a fragment called angiotensin I (AI). AI itself has only slight pharamacologic activity but, after additional cleavage by a second enzyme, angiotensin converting enzyme (ACE), forms the potent molecule angiotensin II (AII). The major pharmacological effects of AII are vasoconstriction and stimulation of the adrenal cortex to release aldosterone, a hormone which causes sodium retention. Sodium retention causes blood volume to increase, which leads to hypertension. AII is cleaved by an aminopeptidase to form angiotensin III (AIII), which, compared to AII, is a less potent vasoconstrictor but a more potent inducer of aldosterone release.
The renin-angiotensin system has been modulated or manipulated, in the past, with ACE inhibitors. However, ACE acts on several substrates other than angiotensin I (AI), most notably the kinins which cause such undesirable side effects as pain, "leaky" capillaries, prostaglandin release and a variety of behavorial and neurologic effects. Further, ACE inhibition leads to the accumulation of AI. Although AI has much less vasoconstrictor activity than AII, its presence may negate some of the hypotensive effects of the blockade of AII synthesis.
Inhibition of other targets in the renin-angiotensin system such as AII with compounds such as saralasin can block AII activity, but would leave unimpaired and perhaps enhance the hypertensive effects of AIII.
Inhibitors of renin have been sought as agents for control of hypertension and as diagnostic agents for identification of cases of hypertension due to renin excess.
In efforts to identify compounds which inhibit renin, compounds have been prepared which mimic angiotensinogen, the natural substrate for renin. In particular, compounds have been prepared which incorporate mimics of the dipeptide sequence of angiotensinogen preceeding the renin cleavage site (i.e., mimics of Phe-His) and which also incorporate non-cleavable mimics of the renin cleavage site of angiotensinogen (i.e., Leu-Val). Compounds comprising mimics of both portions of angiotensinogen bind to renin, but are not cleaved. Thus, renin is inhibited from acting on its natural substrate.